(1) Field of the Invention
The present invention relates to a direct-heated flow measuring apparatus having a film resistor which serves as a temperature detecting means as well as an electric heater. Such a direct-heated flow measuring apparatus can be used, for example, for measuring the flow rate of engine intake air.
(2) Description of the Related Art
Generally, in an internal combustion engine, the amount of intaker air is one of the most important parameters for controlling the fuel injection amount, ignition timing, and the like. A gas-flow measuring apparatus, i.e., an airflow meter, is provided for measuring the same. One of the more common prior art airflow meters is the vane-type, which is, however, disadvantageous in scale, response speed characteristics, and the like. Recently airflow meters having temperature-dependent resistors have been developed, which are advantageous in scale, response speed characteristics, and the like.
There are two types of airflow meters having temperature-dependent resistors, i.e. the heater-type and direct-heated type. The heater-type airflow meter may consist of an electric heater resistor provided in an intake-air passage of an engine and two temperature-dependent resistors arranged on the upstream and downstream sides of the electric heater resistor. In this case, the temperature-dependent resistor on the downstream side is used for detecting the temperature of air heated by the heater resistor, while the temperature-dependent resistor on the upstream side is used for detecting the temperature of non-heated air.
In the heater-type airflow meter, the current flowing through the heater resistor is controlled for a constant difference in temperature between the two temperature-dependent resistors, thereby detecting the voltage applied to the heater resistor as the mass flow rate of air. In this heater-type airflow meter, if no temperature-dependent resistor upstream is provided and the current of the heater resistor is controlled for a constant temperature of the downstream temperature-dependent resistor, the voltage applied to the heater resistor is detected as the volume flow rate of air.
Also, the heater-type airflow meter can be controlled by a trigger pulse. That is, when such a system is employed, a trigger pulse is given to initiate heating of the heater resistor. Then, the heating of the heater resistor continues until a constant difference in temperature between the two temperature-dependent resistors is generated, or until the downstream temperature-dependent resistor reaches a constant value. In this case, the heating time period is detected as the mass flow rate of air of the volume flow rate of air. Such a trigger pulse control has an advantage in that the power dissipation is good.
On the other hand, the direct-heated type airflow meter may consist of a film resistor which serves not only as an electric heater, but also a temperature-detecting means for detecting the temperature of the heated air. Also, the direct-heated type airflow meter may consist of a temperature-dependent resistor for detecting the temperature of non-heated air. Thus, the current flowing through the film resistor is controlled for a constant difference in temperature between the film resistor and the temperature dependent resistor, thereby detecting the voltage applied to the film resistor as the mass flow rate of air. In this direct-heated type airflow meter, too, if no temperature-dependent resistor is provided and the current of the heater resistor is controlled for a constant temperature of the film resistor, the voltage applied to the film resistor is detected as the volume flow rate of air. Note that the above-mentioned trigger pulse control is possible also in the direct-heated airflow meter.
Since the film resistor of the direct-heated type airflow meter serves as a temperature-detecting means for heated air, that is, no additional temperature detecting means for heated air is necessary, the direct-heated type airflow meter is smaller in size than the heater-type airflow meter.
In the direct-heated type airflow meter, one of the film resistors may consist of heat-resistant resin such as a polyimid film, and a metal foil adhered thereto by adhesives, and another of the film resistors may consist of an insulating substrate such as a ceramic or monocrystalline silicon. In the former film resistor, although the heat-resistant temperature of the polyimid film is about 400.degree. C., the heat-resistant temperature of adhesives for high temperatures is about 250.degree. C. However, as shown in FIG. 1, if this airflow meter is used in an internal combusion engine, the temperature of the film resistor is normally controlled within a range of the ambient temperature +(100.degree. to 150.degree. C.), i.e., within a range of 60.degree. C. to 210.degree. C. However, in an abnormal state such as backfiring, the ambient temperature, which is the same as that of the temperature-dependent resistor, may be higher than 200.degree. C., and as a result, the temperature of the film resistor may be higher than 300.degree. C. Thus, during an abnormal state, the film resistor may be exposed to a temperature higher than 250.degree. C. for a long time, so that the polyimid film as well as the adhesives are contracted or deformed, thereby reducing the accuracy of the measured flow rate of air. Also, in an airflow meter using the latter film resistor, the resistance of the resistance pattern may be changed due to the distortion of the substrate such as a ceramic or monocrystalline silicon during an abnormal state, thereby also reducing the accuracy of the measured flow rate of air.